Yes, I was looking for something a little faster, but that's a good thought.
Actually, I have one of those things, but haven't pointed the usrp at it
yet. As a side note, I was thinking about writing a demodulator for
those.. I found the FCC filings for them which described how they worked.
-j
Weber, Michael J. (US SSA) wrote:
Most wireless weather sensors transmit a half-second burst or so every
40-60 seconds around 433MHz... you can pick up a remote temp/humidity
sensor from Radio Shack if you or your neighbors don't have one already.
I've also seen wireless thermometer setups in Home Depot, Target,
K-mart, and the like.
Or, were you looking for something a little faster?
Mike
-----Original Message-----
From:
[EMAIL PROTECTED]
[mailto:discuss-gnuradio-bounces+michael.weber=baesystems.com@
gnu.org] On Behalf Of James Cooley
Sent: Sunday, July 10, 2005 4:18 PM
To: James Cooley
Cc: Eric Blossom; discuss-gnuradio@gnu.org
Subject: Re: [Discuss-gnuradio] FFT of FFTs
Any more suggestions on spotting periodic content? (as below?)
Chiefly, can anyone think of something that I might be able to
definitely pick out within TVRX range?
-jamie
James Cooley wrote:
OK, I'm just getting around to trying these suggestions... First, re
Eric's suggestions below.
A few quick questions about this.
I'm interested in tuning to a frequency (with usrp/frontend) then
examining a single stream for periodic content. I've tried this but
not exactly sure what I'm seeing.
The questions are, have I set this up properly? I have intended to
grab whatever signal is at 50kHz (arbitrary) above what
we're tuned to
and analyze that.
How do I best select the feed forward and feed back filter
coefficients? (I ended up using the tool as written in the
comments).
Also, any suggestions on things to look at within the TVRX
range that
should definitely test this out? (I think that GSM (TDMA)
bands are a
bit out of range... up around 900MHz). I wondered if I
could pick out
the periodicity of NTSC scans perhaps (maybe I'd need like a test
pattern, eh? to guarantee that the scan is periodic and non
changing
long enough for me to try it).
# xlating filter
adnl_decim = 1
taps = [1.0]
shift = -50e3
capture_rate = usrp_rate
channel_coeffs = gr.firdes.low_pass (1.0,
# gain
capture_rate, #
sampling rate
200e3,
# low
pass cutoff freq
200e3,
# width
of trans. band
gr.firdes.WIN_HAMMING)
xlate_filt =
gr.freq_xlating_fir_filter_ccf(adnl_decim,
channel_coeffs,
shift,
capture_rate)
# complex to magnitude
cplx_to_mag = gr.complex_to_mag()
#
# Filter Coeffs correspond to butterworth iir
low pass filter
# passband 0 - 1000 Hz
# Order 1
#
#
http://www.dsptutor.freeuk.com/IIRFilterDesign/IIRFilterDesign.html
#
fbtaps = [0.29289326, 0.29289326]
fftaps = [1.0, -0.41421357]
iir_low_pass = gr.iir_filter_ffd(fftaps, fbtaps)
# fft
occ_fft = fftsink.fft_sink_f (self, panel,
title="Occupancy
FFT",
fft_size = 512,
sample_rate=usrp_rate,
baseband_freq=0)
self.connect (src, cplx_to_mag, iir_low_pass, occ_fft)
Eric Blossom wrote:
On Sat, Jun 18, 2005 at 08:18:38PM -0400, James Cooley wrote:
Hi all,
I'm trying to take an FFT of an FFT.... Basically, I want
to tune to
a signal, and for a given RF frequency, try to spot
periodic usage
of that frequency. Is this possible? What I have now is
hopelessly
slow, so I'm not really sure if I've got it right.
Hi Jamie,
Here are a couple of suggestions. If there are relatively few
frequencies that you want to observe for periodic
occupancy, I would
suggest extracting the frequency bands of interest using a
gr.freq_xlating_fir_filter for each one. If there are
lots of them,
and they are evenly spaced, then the dft filterbank is
what you want
to split them out (blksimpl/filterbank.py).
Once you've got your individual streams of signals, for each one I
would compute an estimate of whether it is occupied. You could do
this by computing the magnitude of the stream
(gr.complex_to_mag) and
then low pass filtering that with a gr.iir_filter,
possibly followed
by a limiter (which would need to be written). At this point, for
each of your input streams, you have an output stream that is
effectively a stream of 1's and 0's, where 1 means "is occupied".
Then run each of those streams into it's own FFT. Point
this whole
pipeline at some kind of TDMA input (GSM basestation?) and
you ought
to see the slots (assuming the basestation isn't driving all the
slots all the time).
Eric
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